The Savannah River Laboratory is using existing techniques and developing new methodology to determine the environmental impact of the Savannah River Plant with regard to 129I and 99Tc. 129I is determined by neutron activation after the method of Brauer. Activation products are quantified by gamma-ray spectroscopy [germanium (lithium)] following chemical isolation. 125I is used as a yield tracer. 129I amounts as low as 3.8 fCi can be determined with 30-min counting times. An isotope dilution method for 99Tc based on a three-stage surface ionization mass spectrometer is being developed. Its chemical isolation scheme ends with the technetium loaded on a single ion-exchange bead for enhanced mass-spectrometric sensitivity. 97Tc will be used as a yield tracer. A lower limit of 0.2 fCi is sought. A modified method using liquid scintillation counting has determined 99Tc in some aqueous samples. These methods have confirmed that 129I and 99Tc can be highly mobile in the aqueous environment, establishing the need for monitoring.